And some libertarians think that most liberals support slavery by supporting universal healthcare or thinking bakers should have to bake cakes for anyone who wants.

There are a lot of bad framings, and "all conservatives think slavery is ok" is a terrible one

But they're still thinking of it as evil.

Slavery is completely anti-ethical to libertarianism.

True. I would still argue that they should get their priorities straight. Slavery is much worse than taxation and even mentioning them in the same context is not OK.

This doesn't make sense. Slavery is obviously a particularly bad form of taxation.

I hope you are trying to be funny.

This isn't funny. I'd argue that it is true, though. Taxation, as an idea, is simply the idea of taking something from someone 'for the common welfare' (ie, other people's well being).

Taking everything, including choice, (in essence slavery) could be called a 'tax' using the above definition.

To compare taxes (as they are presently levied against any first world population) to slavery is quite a stretch, though.

No, I’m saying that even if you think taxes are the only evil possible, slavery is effectively a 100% tax. So there is no possible sense in which it’s less bad.

Is it? What if it is entered into voluntarily?

Your rights are inalienable. This means that you cannot transfer them, even voluntarily. They remain your rights. Slave contracts are worthless for this reason.

Who defines which rights you have and which ones are inalienable?

Rights are inherent in our nature. They are discovered.

Society.